A prime component of any electrical power grid system is the transformer. Transformers are critical to the power grid system and expensive to replace. Therefore, it is a high priority of utility companies and power companies to protect them against internal and external events that may cause damage or failure. One such event is internal arcing, which in an oil-filled transformer, can instantly vaporize the surrounding oil. The oil vaporization can lead to high gas pressures that may rupture the tank.
To prevent or minimize the damage caused by internal arcing, rapid changes in gas pressure must be monitored and detected. When rapid gas pressure changes are detected, the transformer must be taken offline (i.e., the transformer must be turned off). However, external faults to the transformer (e.g., transmission line faults or geomagnetic induced currents) can also cause a pressure rise to occur inside the tank due to an increase in winding heating. It is undesirable to have external faults to the transformer trip the transformer offline.
To differentiate between internal and external faults, a system should be employed that measures variations in time and pressure intensity. The measurement of these variations is significant since a rapid pressure rise system may be installed in the gas space above the cooling oil or in the cooling oil itself. An event that causes a change in pressure has a greater magnitude in the oil space versus the gas space. The gas space mutes the intensity of the pressure change since the gas is more compressible than the oil. Therefore, the response of a rapid pressure rise detection system in the gas space must be faster in order to compensate for this change in transfer intensity. A significant technological challenge to be overcome with a rapid pressure rise detection device is to react to events that will cause further damage and not react to any extraneous stimulus and unnecessarily take the transformer offline. It is problematic to unnecessarily take a transformer offline because a utility company must deploy repair crews anytime a transformer is taken offline.
Historically, mechanical rapid pressure rise relays (“RPRRs”) have been used (e.g., U.S. Pat. No. 4,074,096) as a protection scheme for oil filled power transformers. These devices may use bellows or other sensitive mechanical elements to sense a rate of pressure change and then actuate switches used for tripping a transformer offline. The response curves for the mechanical RPRRs described by U.S. Pat. No. 4,074,096 are shown in FIG. 1. The key features of these curves are as follows: (1) the relay must not trip a transformer offline for any pressure changes of less than 0.22 psi per second; (2) the time for a relay mounted in the gas space of a transformer to operate for a pressure rate of 10 psi per second is 0.178 seconds; and (3) for a relay mounted in the cooling oil, the time to operate the relay for a pressure rate of 10 psi per second is 0.267 seconds. Significantly, in the mechanical RPRRs, these responses to pressure changes are fixed.
The actual pressure rate of rise due to an external fault in an oil-filled transformer is dependent on many different factors (e.g., transformer design, electrical impedances, transformer location, and the like). Therefore, it is desirable for a rapid pressure rise detection system to have the ability to adjust the sensitivity level or response of the RPRR after the system has been installed.
In this manner, U.S. Pat. No. 4,823,224 incorporates an analog rapid pressure rise circuit for detecting changes in pressure using discrete electronic components and electromechanical relays for tripping a transformer offline. This analog rapid pressure rise circuit allows for the adjustment of the response of the RPRR. Accordingly, this analog rapid pressure rise circuit can have its sensitivity level adjusted after its installation to have the same response as a previously installed relay. The range of response curves described by U.S. Pat. No. 4,823,224 matched both sets of curves previously disclosed by U.S. Pat. No. 4,074,096 as shown in FIG. 1.
The response curves in FIG. 1 from the U.S. Pat. No. 4,074,096, which are further explained in U.S. Pat. No. 4,823,224, follow the equation of:t=−T(ln[1−((dp Max)/(dp/dt))])
where: ‘T’ is the time constant of the Time delay circuit;
‘dp Max’ is the maximum rate of change of pressure which will not cause a relay to operate (in this case 0.22 psi/sec);
‘dp/dt’ is the rate of pressure change; and
‘t’ is the time of operate.
Using the cardinal points from the curve and solving for ‘T’ gives ‘T’=8 seconds for the RPRR mounted in the gas space and 12 seconds for a relay mounted in the cooling oil. For any RPRR device, regardless of whether it is mechanical or electrical, once it has detected a pressure rate of rise greater than the sensitivity curve setting it should take the transformer offline within one electrical cycle (60 Hz=16 mSec; 50 Hz=20 mSec).
Mechanical RPRRs have been around for many years and are still used extensively on transformers. However, mechanical RPRRs have some significant limitations. First, mechanical RPRRs are not adjustable in the field. Accordingly, the response curves of mechanical RPRRs depend on the tolerance of the mechanical elements and how they are manufactured. Once mechanical RPRRs are built, they have one and only one sensitivity setting. As such, a mechanical RPRR built for the gas space will not work in the cooling oil and a mechanical RPRR built for the cooling oil will not work in the gas space. Additionally, a mechanical device requires an external latching switch to capture a momentary function of the unit.
While electronic analog RPRR devices have some sensitivity adjustment, this adjustment is limited. The electronic boards are built with a limited set of response curves and the adjustment only allows the user to switch from one preset, fixed curve to the next preset, fixed curve. Even with this minor adjustment capability, present electronic analog RPRRs cannot discern between internal pressure faults and some normal transformer operations or external events, such as seismic events. There is a continuing need in the art for a rapid pressure rise detection system that can overcome these limitations.
Notably, all of the subject matter discussed in this section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in this section. Accordingly, any recognition of problems in the prior art discussed in this section or associated with such subject matter should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion of any subject matter in this section should be treated as part of the identification of the technological problem to be overcome, which in and of itself may also be inventive.